Field of the Invention
This invention relates generally to a fuel cell stack having improved end cell performance and, more particularly, to a fuel cell stack having improved end cell performance by providing a diffusion media for the end cells that has less of an intrusion into the flow field channels than the diffusion media for the flow channels of the other cells so as to increase the flow rate through the flow channels in the end cells relative to the flow rate through the flow channels in the other cells.
Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The membrane within a fuel cell needs to have a certain relative humidity so that the ionic resistance across the membrane is low enough to effectively conduct protons. This humidification may come from the stack water by-product or external humidification. The flow of the reactant gas through the flow channels has a drying effect on the membrane, most noticeably at an inlet of the flow channels. Also, the accumulation of water droplets within the flow channels from the membrane hydration and water by-product could prevent reactant gas from flowing therethrough, and cause the cell to fail, thus affecting the stack stability. The accumulation of water in the reactant gas flow channels is particularly troublesome at low stack output loads.
The end cells in a fuel cell stack typically have a lower performance as a result of cell stability than the other cells in the stack. Particularly, the end cells are more exposed to ambient temperature, and thus have a temperature gradient that causes them to operate at a lower temperature as a result of convective heat losses. Because the end cells are typically cooler than the other cells in the stack, water vapor more easily condenses into liquid water so that the end cells have a higher relative humidity, which causes water droplets to more readily form in the flow channels of the end cells. Also, at low stack loads the temperature of the cooling fluid is reduced, which reduces the temperature of the stack and typically increases the relative humidity of the reactant gas flow.
End cell stability can be illustrated by the following example. A fuel cell stack may be operating at a certain current density, such as 0.6 A/cm2, which provides a cell voltage of about 0.7 volts. The flow rate of reactant gas through the reactant gas flow channels to produce this current density is sufficient to force accumulated water in the flow channels out of the flow channels. If the current density of the stack is reduced to 0.1 A/cm2, such as for vehicle idle, the cell voltage increases to about 0.85 volts, and the flow rate of reactant gas through the flow channels is significantly reduced. Because the end cells are at a lower temperature, more water will condense in the end cell flow channels increasing channel blockage. Because the channels may be blocked with water, the reactant gas may be diverted to the flow channels of other fuel cells, which causes the voltage of the end cells to become unstable.
It is known in the art to heat the end cells with resistive heaters positioned between the unipolar plate and the MEA so as to compensate for convective heat losses. These known systems typically attempted to maintain the end cell temperature the same as the other cells in the stack by monitoring the temperature of the cooling fluid out of the stack. However, end cell stability is still a problem even with the addition of such heaters.